Studies are underway to identify chemical neurotransmitters in retinal neurons and other neurons in visual pathways, to examine the development of neurotransmitter systems in retina, and to determine the role of chemical neurotransmitters in processing visual information. Biochemical and anatomical techniques are used in these studies. Two systems are currently being investigated: (1) the relatively simple visual system of Limulus polyphemus and (2) developing mammalian retinal neurons in intact retinas and in monolayer cell culture. Work with the simple visual system has led to the first identification of an efferent chemical neurotransmitter probably involved in control of circadian changes in photoreceptor cell sensitivity and of circadian photoreceptor cell membrane turnover. Studies of the intact developing mammalian retina have shown that some neurotransmitter development occurs very early in retinal development. Information on the development of the intact mammalian retina will form the basis for future studies of mechanisms controlling normal and abnormal development of retinas using the cell culture system.